electronic measurements and instrumentation · 2019. 10. 23. · electronic measurements and...

Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

UNIT-IV

Transducers

Transducer is a device that converts energy in one form of energy to another form of energy.

This converts non-electrical quantity into electrical quantity.

A transducer is defined as a device that receives energy from one system and transmits it to

another, often in a different form.

Broadly defined, the transducer is a device capable of being actuated by an energizing

input from one or more transmission media and in turn generating a related signal to one or

more transmission systems. It provides a usable output in response to a specified input

measurand, which may be a physical or mechanical quantity, property, or conditions. The

energy transmitted by these systems may be electrical, mechanical or acoustical.

The nature of electrical output from the transducer depends on the basic principle

involved in the design. The output may be analog, digital or frequency modulated.

Basically, there are two types of transducers, electrical, and mechanical. Transducer is a

device that converts energy in one form of energy to another form of energy. This converts

non-electrical quantity into electrical quantity.

Electrical Transducer Definition

An electrical transducer is a sensing device by which the physical, mechanical or optical

quantity to be measured is transformed directly by a suitable mechanism into an electrical

voltage/current proportional to the input measurand.

An electrical transducer must have the following parameters:

1. Linearity: The relationship between a physical parameter and the resulting electrical signal

must be linear.

2. Sensitivity: This is defined as the electrical output per unit change in the physical parameter

(for example V/°C for a temperature sensor). High sensitivity is generally desirable for a

transducer.

3. Dynamic Range: The operating range of the transducer should be wide, to permit its use

under a wide range of measurement conditions.

4. Repeatability: The input/output relationship for a transducer should be predictable over a

long period of time.

5. Physical Size: it must have minimal weight and volume, so that its presence in

the measurement system does not disturb the existing conditions.

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Advantages of Electrical Transducer

The main advantages of electrical transducer (conversion of physical quantity into electrical

quantities) are as follows:

1. Electrical amplification and attenuation can be easily done.

2. Mass-inertia effects are minimised.

3. Effects of friction are minimised.

4. The output can be indicated and recorded remotely at a distance from the sensing medium.

5. The output can be modified to meet the requirements of the indicating or controlling units.

The signal magnitude can be related in terms of the voltage current. (The analog signal

information can be converted in to pulse or frequency information. Since output can be

modified, modulated or amplified at will, the output signal can be easily used for recording on

any suitable multichannel recording device.)

6. The signal can be conditioned or mixed to obtain any combination with outputs of similar

transducers or control signals.

7. The electrical or electronic system can be controlled with a very small power level.

8. The electrical output can be easily used, transmitted and processed for the purpose of

measurement.

Electrical transducer can be broadly classified into two major categories,

Classification of transducers

• Primary and Secondary Transducers

• Analog and Digital Transducers

• Active and Passive Transducers

• Transducers and Inverse Transducers

Primary and Secondary Transducers

When the input signal is directly sensed by the transducer and physical phenomenon is

converted into the electrical form directly then such a transducer is called the primary

transducer.

Example: The thermistor senses the temperature directly and causes the change in resistance

with the change in temperature.

When the input signal is sensed first by some detector or sensor and then its output being of

some form other than input signals is given as input to a transducer for conversion into

electrical form, then such a transducer falls in the category of secondary transducers.



For example, in case of pressure measurement, bourdon tube is a primary sensor which

converts pressure first into displacement, and then the displacement is converted into an

output voltage by an LVDT.

Analog and Digital Transducers

Analog transducer converts input signal into output signal, which is a continuous function of

time such as thermistor, strain gauge, LVDT, thermo-couple etc.

Digital transducer converts input signal into the output signal of the form of pulse e.g. it gives

discrete output.

Transducers and Inverse Transducers

Transducer, as already defined, is a device that converts a non-electrical quantity into an

electrical quantity.

Normally a transducer and associated circuit has a non-electrical input and an electrical output,

for example a thermo-couple, photoconductive cell, pressure gauge, strain gauge etc.

An inverse transducer is a device that converts an electrical quantity into a non-electrical

quantity.

Example: piezoelectric oscillator

Active and Passive Transducers

An active transducer generates an electrical signal directly in response to the physical

parameter and does not require an external power source for its operation. Active transducers

are self generating devices, which operate under energy conversion principle and generate an

equivalent output signal (for example from pressure to charge or temperature to electrical

potential).

Typical example of active transducers are piezo electric sensors (for generation of charge

corresponding to pressure) and photo voltaic cells (for generation of voltage in response to

illumination).

Passive transducer operates under energy controlling principles, which makes it

necessary to use an external electrical source with them. They depend upon the change in an

electrical parameter (R, L and C).

Typical example are strain gauges (for resistance change in response to pressure), and

thermistors (for resistance change corresponding to temperature variations).

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Electrical transducer is used mostly to measure non-electrical quantities. For this

purpose a detector or sensing element is used, which converts the physical quantity into a

displacement. This displacement actuates an electric transducer, which acts as a secondary

transducer and gives an output that is electrical in nature. This electrical quantity is measured

by the standard method used for electrical measurement. The electrical signals may be

current, voltage, or frequency; their production is based on R, L and C effects.

A transducer which converts a non-electrical quantity into an analog electrical signal

may be considered as consisting of two parts, the sensing element, and the transduction

element.

The sensing or detector element is that part of a transducer which responds to a physical

phenomenon or to a change in a physical phenomenon. The response of the sensing element

must be closely related to the physical phenomenon.

The transduction element transforms the output of a sensing element to an electrical output.

This, in a way, acts as a secondary transducer.

Transducers may be further classified into different categories depending upon the

principle employed by their transduction elements to convert physical phenomena into output

electrical signals.

Selecting a Transducer

The transducer or sensor has to be physically compatible with its intended application. The

following should be considered while selecting a transducer.

1. Operating range: Chosen to maintain range requirements and good

2. Sensitivity: Chosen to allow sufficient output.

3. Frequency response and resonant frequency: Flat over the entire desired range.

4. Environmental compatibility: Temperature range, corrosive fluids, pressure, shocks,

interaction, size and mounting restrictions

5. Minimum sensitivity: To expected stimulus, other than the measurand.

6. Accuracy: Repeatability and calibration errors as well as errors expected due to sensitivity

to other stimuli.

7. Usage and ruggedness: Ruggedness, both of mechanical and electrical intensities versus

size and weight.

8. Electrical parameters: Length and type of cable required, signal to noise ratio when

combined with amplifiers, and frequency response limitations.

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Resistive Transducer

Resistive Transducers are those in which the resistance changes due to a change in some

physical phenomenon. The change in the value of the resistance with a change in the length of

the conductor can be used to measure displacement.

Strain gauges work on the principle that the resistance of a conductor or semiconductor

changes when strained. This can be used for the measurement of displacement, force and

pressure.

The resistivity of materials changes with changes in temperature. This property can be used

for the measurement of temperature.

Potentiometer (Displacement Transducer)

A resistive potentiometer (pot) consists of a resistance element provided with a sliding

contact, called a wiper. The motion of the sliding contact may be translatory or rotational.

Some have a combination of both, with resistive elements in the form of a helix, as shown in

Fig. 13.1(c). They are known as helipots.

Translatory resistive elements, as shown in Fig. 13.1(a), are linear (straight) devices.

Rotational resistive devices are circular and are used for the measurement of angular

displacement, as shown in Fig. 13.1(b).

Helical resistive elements are multi turn rotational devices which can be used for the

measurement of either translatory or rotational motion. A potentiometer is a passive

transducer since it requires an external power source for its operation.

Advantage of Potentiometers

1. They are inexpensive.

2. Simple to operate and are very useful for applications where the requirements are not

particularly severe.

3. They are useful for the measurement of large amplitudes of displacement.

4. Electrical efficiency is very high, and they provide sufficient output to allow control

operations.

Disadvantages of Potentiometers

1. When using a linear potentiometer, a large force is required to move the sliding contacts.

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2. The sliding contacts can wear out, become misaligned and generate noise.

Strain gauges

The strain gauge is an example of a passive transducer that uses electric resistance

variation in wires to sense the strain produced by a force on wires. It is a very versatile

detector and transducer for measuring weight, pressure, mechanical force, or displacement.

The construction of a bonded strain gauge (see figure) shows a fine wire element looped

back and forth on a mounting plate, which is usually cemented to the member undergoing

stress. A tensile stress tends to elongate the wire and thereby increase its length and decrease

its cross-sectional area.

Bonded type strain gauges are three types, namely

1. Wire Strain Gauges

2. Foil Strain Gauge

3. Semiconductor Strain Gauge

1. Wire Strain Gauges:

Wire Strain Gauges has three types namely,

1. Grid type

2. Rossette type

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3. Torque type

4. Helical type

The grid arrangement of the wire element in a bonded strain gauge creates a problem

not encountered in the use of unbonded strain gauges. To be useful as a strain gauge, the wire

element must measure strain along one axis. Therefore complete and accurate analysis of

strain in a rigid member is impossible, unless the direction and magnitude of stress are known.

The measuring axis of a strain gauge is its longitudinal axis, which is parallel to the wire

element, as shown in Fig. 13.6.

When a strain occurs in the member being measured, along the transverse axis of the

gauge, it also affects the strain being measured parallel to the longitudinal axis. This introduces

an error in the response of the gauge.

In most applications, some degree of strain is present along the transverse axis and the

transverse sensitivity must be considered in the final gauge output. Transverse sensitivity

cannot be completely eliminated, and in highly accurate measurements the resultant gauge

error must be compensated for.

If the axis of the strain in a component is unknown, Strain Gauge Transducer Types may

be used to determine the exact direction. The standard procedure is to place several gauges at

a point on the member’s surface, with known angles between them. The magnitude of strain in

each individual gauge is measured, and used in the geometrical determination of the strain in

the member.

Characteristics of a resistance wire strain gauge

• The Strain Gauge Transducer Types should have a high value of gauge factor (a high

value of gauge factor indicates a large change in resistance for particular strain, implying high

sensitivity).

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• The resistance of the strain gauge should be as high as possible, since this minimizes the

effects of undesirable variations of resistance in the measurement circuit. A high resistance

value results in lower sensitivity.

• The strain gauge should have a low resistance temperature coefficient.

• The strain gauge should not have hysteresis effects in its response

• The variation in resistance should be a linear function of the strain.

• Strain gauges are frequently used for dynamic measurements and hence their

frequency response should be good.

• Leads used must be of materials which have low and stable resistivity and low

resistance temperature coefficient.

Foil Strain Gauge

This class of strain gauges is an extension of the resistance wire strain gauge. The strain

is sensed with the help of a metal foil. The metals and alloys used for the foil and wire are

nichrome, constantan (Ni + Cu), isoelastic (Ni + Cr + Mo), nickel and platinum.

Foil gauges have a much greater dissipation capacity than wire wound gauges, on

account of their larger surface area for the same volume. For this reason, they can be used for a

higher operating temperature range. Also, the large surface area of foil gauges leads to better

bonding.

Foil type Strain Gauge Transducer Types have similar characteristics to wire strain

gauges. Their gauge factors are typically the same.

The advantage of foil type Strain Gauge Transducer Types is that they can be fabricated

on a large scale, and in any shape. The foil can also be etched on a carrier.

Etched foil gauge construction consists of first bonding a layer of strain sensitive

material to a thin sheet of paper or bakelite. The portion of the metal to be used as the wire

element is covered with appropriate masking material, and an etching solution is applied to the

unit. The solution removes that portion of the metal which is not masked, leaving the desired

grid structure intact.

This method of construction enables etched foil strain gauges to be made thinner than

comparable wire units, as shown in Fig. 13.9. This characteristic, together with a greater degree

of flexibility, allows the etched foil to be mounted in more remote and restricted places and on

a wide range of curved surfaces. The resistance value of commercially available foil gauges is

between 50 and 1000 Ω















Semiconductor Strain Gauge

To have a high sensitivity, a high value of gauge factor is desirable. A high gauge factor means relatively

higher change in resistance, which can be easily measured with a good degree of accuracy.

Semiconductor strain gauges are used when a very high gauge factor is required. They have a gauge factor

50 times as high as wire strain gauges. The resistance of the semiconductor changes with change in applied

strain.

Semiconductor strain gauges depend for their action upon the piezo resistive effect, i.e. change in value of

the resistance due to change in resistivity, unlike metallic gauges where change in resistance is mainly due

to the change in dimension when strained. Semiconductor materials such as germanium and silicon are

used as resistive materials.

A typical strain gauge consists of a strain material and leads that are placed in a protective box, as shown in

Fig. 13.10. Semiconductor wafer or filaments which have a thickness of 0.05 mm are used. They are

bonded on suitable insulating substrates, such as Teflon.

Gold leads are generally used for making contacts. These strain gauges can be fabricated along with an IC

Op Amp which can act as a pressure sensitive transducer. The large gauge factor is accompanied by a

thermal rate of change of resistance approximately 50 times higher than that for resistive gauges. Hence,

a semiconductor strain gauge is as stable as the metallic type, but has a much higher output.

Simple temperature compensation methods can be applied to semiconductor strain gauges, so that small

values of strain, that is micro strains, can also be measured.

Advantages of Semiconductor Strain Gauge



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1. Semiconductor strain gauges have a high gauge factor of about + 130. This allows measurement of very

small strains, of the order of 0.01 micro

2. Hysteresis characteristics of semiconductor strain gauges are excellent, e. less than 0.05%.

3. Life in excess of 10 x 106 operations and a frequency response of 1012 HZ.

4. Semiconductor strain gauges can be very small in size, ranging in length from 0.7 to 7.0 mm.

Disadvantages of Semiconductor Strain Gauge

1. They are very sensitive to changes in temperature.

2. Linearity of semiconductor strain gauges is poor.

3. They are more expensive.

Temperature Transducers

1. Resistance Temperature Detectors (RTD)

2. Thermocouples

3. Thermistor

Resistance Temperature Detector (RTD)

Detectors of wire resistance temperature common employ platinum, nickel or resistance wire elements,

whose resistance variation with temperature has high intrinsic accuracy. They are available in many

configurations and size and as shielded or open units for both immersion and surface applications.

The relationship between temperature and resistance of conductors can be calculated from the equation:

Thermistor

A thermistor is a semiconductor made by sintering mixtures of metallic oxide, such as oxides of

manganese, nickel, cobalt, copper and uranium.

Termistors have negative temperature coefficient (NTC). That is, their resistance decreases as their

temperature rises.



Types of thermistor Resistance

Disc 1 to 1MΩ

Washer 1 to 50kΩ

Rod high resistance

This figure shows resistance versus temperature for a family thermistor. The resistance value marked at

the bottom end of each curve is a value at 250C

The resistance decreases as their temperature rises-NTC

Advantages of thermistor

• Small size and low cost

• Fast response over narrow temperature range

• Good sensitivity in Negative Temperature Coefficient (NTC) region

• Cold junction compensation not required due to dependence of resistance on absolute

temperature.

• Contact and lead resistance problems not encountered due to large resistance

Limitations of thermistor

• Non linearity in resistance vs temperature characteristics

• Unsuitable for wide temperature range

• Very low excitation current to avoids self heating

• Need of shielded power lines, filters, etc due to high resistance

Thermocouples

It consists of two wires of different metals are joined together at one end, a temperature difference

between this end and the other end of wires produces a voltage between the wires. The magnitude of this

voltage depends on the materials used for the wires and the amount of temperature difference between

the joined ends and the other ends.



A current will circulate around a loop made up of two dissimilar metals when the two junctions are at

different temperatures. When this circuit is opened, a voltage appears that is proportional to the observed

seeback current. The Thomson and Peltier emfs originate from the fact that, within conductors, the density

of free charge carriers (electrons and holes) increases with temperature.

• If the temperature of one end of a conductor is raised above that of the other end, excess electrons

from the hot end will diffuse to the cold end. This results in an induced voltage, the Thomson effect that

makes the hot end positive with respect to the cold end.

Conductors made up of different materials have different free-carriers densities even when at the same

temperature. When two dissimilar conductors are joined, electrons will diffuse across the junction from

the conductor with higher electron density. When this happens the conductor losing electrons acquire a

positive voltage with respect to the other conductor. This voltage is called the Peltier emf.

• When the junction is heated a voltage is generated, this is known as seeback effect. The seeback

voltage is linearly proportional for small changes in temperature.

• The magnitude of this voltage depends on the material used for the wires and the amount of

temperature difference between the joined ends and the other ends. The junction of the wires of the

Thermocouple Circuit is called the sensing junction.

• The temperature at this end of the Thermocouple Circuit wire is a reference temperature, this

function is known as the reference, also called as the cold junction.

• When the reference end is terminated by a meter or a recording device, the meter indication will

be proportional to the temperature difference between the hot junction and the reference junction.

• The magnitude of the thermal emf depends on the wire materials used and in the temperature

difference between the junctions.

• Thermal emfs for some common thermocouple materials.

The thermocouple (TC) is a temperature transducer that develops an emf that is a function of the

temperature difference between its hot and cold junctions.

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• Type ‘E’ Thermocouple units use Chromel alloy as the positive electrode and constantan alloy as

the negative electrode.

• Type ‘S’ Thermocouple produces the least output voltage but can be used over greatest

temperature range.

• Type ‘T’ uses copper and constantan.

The emf of the thermocouple:

E = c(T1 – T2) + k(T12 – T22) Where

c and k = constant of the thermocouple materials

T1 = The temperature of the “hot” junction

T2 = The temperature of the “cold” or “reference” junction

Advantages of Thermocouple

• It has rugged construction.

• It has a temperature range from —270 °C-2700 °C.

• Using extension leads and compensating cables, long distances transmission for temperature

measurement is possible.

• Bridge circuits are not required for temperature measurement.

• Comparatively cheaper in cost.

• Calibration checks can be easily performed.

• Thermocouples offer good reproducibility.

• Speed of response is high compared to the filled system thermometer.

• Measurement accuracy is quite good.

Disadvantages of Thermocouple

• Cold junction and other compensation is essential for accurate

• They exhibit non-linearity in the emf versus temperature characteristics.

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• To avoid stray electrical signal pickup, proper separation of extension leads from thermocouple

wire is essential.

• Stray voltage pick-ups are possible.

• In many applications, the signals need to be amplified.

Capacitive Transducer

The capacitance of a parallel plate capacitor is given by

The capacitance of this unit proportional to the amount of the fixed plate that is covered, that shaded by

moving plate. This type of transducer will give sign proportional to curvilinear displacement or angular

velocity.

It consists of a fixed cylinder and a moving cylinder. These pieces are configured so the moving piece fits

inside the fixed piece but insulated from it.

Capacitive Pressure Transducer

A transducer that varies the spacing between surfaces. The dielectric is either air or vacuum. Often used as

Capacitance microphones.



Inductive Transducer

Inductive transducers may be either of the self generating or passive type. The self generating type utilizes

the basic electrical generator principle, i.e, a motion between a conductor and magnetic field induces a

voltage in the conductor (generator action). This relative motion between the field and the conductor is

supplied by changes in the measurand.

An inductive electromechanical transducer is a device that converts physical motion (position change)

into a change in inductance. Transducers of variable inductance type work upon one of the following

principles:

1. Variation of self inductance and Variation of mutual inductance

Inductive transducers are mainly used for the measurement of displacement. The displacement to be

measured is arranged to cause variation in any of three variables

• Number of turns

• Geometric configuration

• Permeability of the magnetic material

Change in Self Inductance with Numbers of Turns

The output may be caused by a change in the number of turns. Figures 13.14(a) and (b) are transducers

used for, the measurement of displacement of linear and angular movement respectively.



Transducer Working on the Principle of Change in Self Inductance with Change in Permeability

Figure 13.15 shows an Inductive Transducer Definition which works on the principle of the variation of

permeability causing a change in self inductance. The iron core is surrounded by a winding. If the iron core

is inside the winding, its permeability is increased, and so is the inductance. When the iron core is moved

out of the winding, the permeability decreases resulting in a reduction of the self inductance of the coil.

This transducer can be used for measuring displacement.

Variable Reluctance Type Transducer

A transducer of the variable type consists of a coil wound on a ferromagnetic core. The displacement which

is to be measured is applied to a ferromagnetic target. The target does not have any physical contact with

the core on which it is mounted. The core and the target are separated by an air gap, as shown in Fig.

13.16(a)

The reluctance of the magnetic path is determined by the size of the air gap. The inductance of the coil

depends upon the reluctance of the magnetic circuits. The self inductance of the coil is given by

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But reluctance of the air gap is given by

Where

lg = length of the air gap

Ag = area of the flux path through air

μo = permeability

Rg is proportional to lg, as μo and Ag are constants.

Hence L is proportional to l/lg, i.e. the self inductance of the coil is inversely proportional to the length of

the air gap.

Differential Output Transducer

• The inductance of one part increases from L to L + ΔL, while that of the other part decreases from L

to L — ΔL. The change is measured as the difference of the two, resulting in an output of 2 ΔL

instead of ΔL, when one winding is used. This increases the sensitivity and also eliminates error.

Linear

Variable Differential Transducer (LVDT)


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The differential transformer is a passive inductive transformer. It is also known as a Linear Variable

Differential Transducer (LVDT).

• The transformer consists of a single primary winding P1 and two secondary windings S1 and S2

wound on a hollow cylindrical former. The secondary windings have an equal number of turns and are

identically placed on either side of the primary windings. The primary winding is connected to an ac source.

• An movable soft iron core slides within the hollow former and therefore affects the magnetic

coupling between the primary and the two secondaries. The displacement to be measured is applied to an

arm attached to the soft iron core.

• When the core is in its normal (null) position, equal voltages are induced in the two secondary

windings. The frequency of the ac applied to the primary winding ranges from 50 Hz to 20 kHz.

• The output voltage of the secondary windings S1 is Es1 and that of secondary winding S2 is Es2.

• In order to convert the output from S1 to S2 into a single voltage signal, the two secondaries S1 and

S2 are connected in series opposition,

• Hence the output voltage of the transducer is the difference of the two voltages. Therefore the

differential output voltage Eo=Es1~Es2.

• When the core is at its normal position, the flux linking with both secondary windings is equal, and

hence equal emfs are induced in them. Hence, at null position Es1 = Es2. Since the output voltage of the

transducer is the difference of the two voltages, the output voltage Eo is zero at null position.




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• Now, if the core is moved to the left of the null position, more flux links with winding S1 and less

with winding S2. Hence, output voltage Es1 of the secondary winding S1 is greater than Es2. The

magnitude of the output voltage of the secondary is then Es1 — Es2, in phase with Es1 (the output

voltage of secondary winding S1).

• Similarly, if the core is moved to the right of the null position, the flux linking with winding S2

becomes greater than that linked with winding S1. This result in Es2 becoming larger than Es1. The

output voltage in this case is Eo = Es2— Es1 and is in phase with Es2.

Advantages

• Linearity: The output voltage of this transducer is practically linear for displacements upto 5 mm (a

linearity of 0.05% is available in commercial LVDTs).

• Infinite resolution: The change in output voltage is stepless. The effective resolution depends more

on the test equipment than on the

• High output: It gives a high output (therefore there is frequently no need for intermediate

amplification devices).

• High sensitivity: The transducer possesses a sensitivity as high as 40 V/mm.

• Ruggedness: These transducers can usually tolerate a high degree of vibration and shock.

• Less friction: There are no sliding contacts.



• Low hysteresis: This transducer has a low hysteresis, hence repeatability is excellent under all

conditions.

• Low power: consumption Most LVDTs consume less than 1 W

Disadvantages

• Large displacements are required for appreciable differential output.

• They are sensitive to stray magnetic fields (but shielding is possible).

• The receiving instrument must be selected to operate on ac signals, or ademodulator network must

be used if a dc output is required.

• The dynamic response is limited mechanically by the mass of the core and electrically by the

applied voltage.

• Temperature also affects the transducer.

Piezoelectric Transducer

• A symmetrical crystalline material such as Quartz, Rochelle salt and Barium titanate produce an emf

when they are placed under stress. This property is used in Piezoelectric Transducer Working

Principle, where a crystal is placed between a solid base and the force-summing member.

• For a Piezoelectrical Transducer element under pressure, part of the energy is, converted to an

electric potential that appears on opposite faces of the element, analogous to a charge on the

plates of a capacitor. The rest of the applied energy is converted to mechanical energy, analogous

to a compressed spring. When the pressure is removed, it returns to its original shape and loses its

electric charge.

From these relationships, the following formulas have been derived for the coupling coefficient K

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• An alternating voltage applied to a crystal causes it to vibrate at its natural resonance frequency.

Since the frequency is a very stable quantity, Piezoelectrical Transducer crystals are principally used in HF

accelerometers.

• The principal disadvantage is that voltage will be generated as long as the pressure applied to the

piezo electric element changes.

Synchros

A Synchro can be an angular position transducer working on Pressure Inductive Transducer principle,

where in a variable coupling between primary and secondary winding is obtained by changing the

relative orientation of the windings. A Synchro appears like an AC motor consisting of a rotor and a

stator. They have a rotor with one or three windings capable of revolving inside a fixed stator. There

are two common types of rotors, the salient pole and the wound rotor.

• The stator has a 3-phase winding with the windings of the 3-phase displaced by 120°. The synchro

may be viewed as a variable coupling transformer.

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The rotor is energized by an ac voltage and coupling between rotor and stator windings varies as a

trigonometric or linear function of the rotor position.

A Synchro system formed by interconnection of the devices called the Synchro transmitter and

Synchro control transmitter is perhaps the most widely used error detector in feedback control system.

It measures and compares two angular displacements and its output voltage is approximately linear

with angular displacement.

When an ac excitation voltage is applied to the rotor, the resultant current produces a magnetic

field and by transformer action induces voltages in the stator coils.

The effective voltage induced in any stator coil depends upon the angular position of the coil axis with

respect to the rotor axis.

Suppose the voltage is V, the coupling between S1 and S2 of the stator and primary (rotor) winding is a

cosine function. In general if the rotor is excited by 50 Hz ac, also called reference voltage, the voltage

induced in any stator winding will be proportional to the cosine of the angle between the rotor axis and

the stator axis.

For example, if a reference voltage V sin ωt excites the rotor of a synchro (R1— R2), the stator

terminals will have a voltage of the following form:

Where θ is the shaft angle.

These voltages are known as Synchro format voltages.

Therefore, the effective voltages in these windings are proportional to cos 60° or they are V/2 each. So

long as the rotor of the transmitter and receiver remains in this position, no current will flow between the

stator windings because of the voltage balance.

When the rotor of the transmitter is moved to a new position, the voltage balance is disturbed or changed.

Assuming that the rotor of the transmitter is moved through 30° as shown in Fig. 13.24(b), the stator

winding voltages of the transmitter will be changed to 0, √3/2 V and √3/2 V respectively.


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Hence, a voltage imbalance occurs between the stator windings of the transmitter and receiver. This

voltage imbalance between the windings causes current to flow between the windings producing a torque

that tends to rotate the rotor of the receiver to a new position where the voltage balance is again restored.

This balance is restored only if the receiver turns through the same angle as the transmitter and also the

direction of rotation is the same as that of the transmitter. Hence a Synchro can be used to determine the

magnitude and direction of angular displacement.

Magnetostrictive Transducer

• Magnetostrictive materials transducer converts magnetic energy to mechanical energy and vice

versa. As a magnetostrictive material is magnetized, it strains; that is it exhibits a change in length

per unit length.



• Conversely, if an external force produces a strain in a magnetostrictive material, the material's

magnetic state will change. This bi-directional coupling between the magnetic and mechanical

states of a magnetostrictive material provides a transduction capability that is used for both

actuation and sensing devices.

Magnetostriction is an inherent material property that will not degrade with time.

Hot Wire Anemometer

Basic Principle:

• When an electrically heated wire is placed in a flowing gas stream, heat is transferred from the wire

to the gas and hence the temperature of the wire reduces, and due to this, the resistance of the

wire also changes. This change in resistance of the wire becomes a measure of flow rate.

There are two methods of measuring flow rate using a anemometer bridge combination namely:

• Constant current method

• Constant temperature method

Constant current method

• The bridge arrangement along with the anemometer has been shown in diagram. The anemometer

is kept in the flowing gas stream to measure flow rate.

A constant current is passed through the sensing wire. That is, the voltage across the bridge circuit

is kept constant, that is, not varied.

Due to the gas flow, heat transfer takes place from the sensing wire to the flowing gas and hence

the temperature of the sensing wire reduces causing a change in the resistance of the sensing wire.

(this change in resistance becomes a measure of flow rate).

Due to this, the galvanometer which was initially at zero position deflects and this deflection of the

galvanometer becomes a measure of flow rate of the gas when calibrated.



Constant temperature method

The bridge arrangement along with the anemometer has been shown in diagram. The anemometer

is kept in the flowing gas stream to measure flow rate.

A current is initially passed through the wire.

Due to the gas flow, heat transfer takes place from the sensing wire to the flowing gas and this

tends to change the temperature and hence the resistance of the wire.

The principle in this method is to maintain the temperature and resistance of the sensing wire at a

constant level. Therefore, the current through the sensing wire is increased to bring the sensing

wire to have its initial resistance and temperature.

The electrical current required in bringing back the resistance and hence the temperature of the

wire to its initial condition becomes a measure of flow rate of the gas when calibrated.

electronic measurements and instrumentation · 2019. 10. 23. · electronic measurements and...

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